Pregnant rats were exposed three times daily to immobilization stress during gestational Days 15-19. The behavior of their offspring was compared with the behavior of offspring from unstressed control mothers. Although the stress procedure decreased the weight gain of mothers during pregnancy, it slightly but significantly increased the weight of their offspring at birth and at weaning. On postnatal Day 10, prenatally stressed pups returned to their home cage more quickly than did prenatally unstressed control pups during a nest odor discrimination task, but no differences between groups in the number of correct responses were found. On postnatal Days 70-72, prenatally stressed offspring showed increased exploratory activity in a complex tunnel maze compared with control offspring. On postnatal Day 80, analgesia induced by stress (swimming for 3 min in cold water) was determined (tail flick latency). The degree of stress-induced analgesia was smaller in prenatally stressed rats than in control rats. These data suggest that the effects of prenatal stress on behavior are most clearly discernable when such animals are confronted with a novel or stressful situation.